Swing path and angle of racket face when hitting DTL off a cross court shot

Some time ago, my son had the privilege of taking a few lessons from an excellent coach (he was a former pro himself and a former Olympic coach for a South American country). The coach said that you should aim at the back (directly behind) of the ball when hitting DTL off a cross court shot. I took that to mean the racket face has to be perpendicular to the sideline which means the swing path naturally is parallel to the sideline.

But, this does not make sense if you believe the rule that the angle of incidence=angle of reflection. In other words, the racket face and the swing path would vary depending on the angle of the cross court shot, the more angled the cross court shot, the more your racket has to face towards the center of the court if you are trying to hit a DTL shot.

What am I missing here? Was the coach wrong in telling my son to aim at the back of the ball or does the force of the swing change the rule of angle of incidence=angle of reflection?

I've actually never thought of this from a physics point of view. But when I want to go down the line - I agree with the coach - hit the back of the ball. I think you are leaving the effect of impact on the ball out of your equation.

If you just leave the racquet there then angles you are taking about will be correct. But if you hit the ball - angles will get changed.

Coach is giving a GUIDELINE, to where to hit the ball.
The player has to compensate for HIS shots, whether he slices, sidespins, topspins, or hits flat, and the WAY he hits the ball, his stroke path and how much across his body.
Every shot is going to be different, and every player does it differently.

Some time ago, my son had the privilege of taking a few lessons from an excellent coach (he was a former pro himself and a former Olympic coach for a South American country). The coach said that you should aim at the back (directly behind) of the ball when hitting DTL off a cross court shot. I took that to mean the racket face has to be perpendicular to the sideline which means the swing path naturally is parallel to the sideline.

But, this does not make sense if you believe the rule that the angle of incidence=angle of reflection. In other words, the racket face and the swing path would vary depending on the angle of the cross court shot, the more angled the cross court shot, the more your racket has to face towards the center of the court if you are trying to hit a DTL shot.

What am I missing here? Was the coach wrong in telling my son to aim at the back of the ball or does the force of the swing change the rule of angle of incidence=angle of reflection?

Harry

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You have to compensate for it. Even at the pro level, you can see DTL misses (ball not entering the court) when an angled CC ball is being hit DTL. The more the ball is moving and spinning away from you, the harder it is to pull it back into the court. The heavier your racket and the faster the swing speed you have, the less necessary it will be to make a CC-pointing angle with your frame.

Some time ago, my son had the privilege of taking a few lessons from an excellent coach (he was a former pro himself and a former Olympic coach for a South American country). The coach said that you should aim at the back (directly behind) of the ball when hitting DTL off a cross court shot. I took that to mean the racket face has to be perpendicular to the sideline which means the swing path naturally is parallel to the sideline.

But, this does not make sense if you believe the rule that the angle of incidence=angle of reflection. In other words, the racket face and the swing path would vary depending on the angle of the cross court shot, the more angled the cross court shot, the more your racket has to face towards the center of the court if you are trying to hit a DTL shot.

What am I missing here? Was the coach wrong in telling my son to aim at the back of the ball or does the force of the swing change the rule of angle of incidence=angle of reflection?

Click to expand...

IMHO, the coach is correct.

Don't get caught up in descriptions like this. Nobody can accurately reproduce any of this stuff on demand. The physics are too precise and fast. It has to come through the brain's learning through trial and error. In general, hitting a straight shot baseline-to-baseline requires primarily hitting the the back of the ball.

I'm not a coach, but I'd probably tell someone something very similar.

Don't get caught up in descriptions like this. Nobody can accurately reproduce any of this stuff on demand. The physics are too precise and fast. It has to come through the brain's learning through trial and error. In general, hitting a straight shot baseline-to-baseline requires primarily hitting the the back of the ball.

I'm not a coach, but I'd probably tell someone something very similar.

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I am inclined to agree with you, except when the CC ball is coming in really fast. I feel like there is a critical speed at which you need to start compensating for the angle of reflection which is off the perpendicular axis from the racket surface. It might depend on a combination of CC angle and ball speed.

Does anyone consciously think about the different swing paths depending on either the CC ball speed or angle if you want to go DTL?

Try practice, then allow your mind to tell your body to adjust.
If you reply meekly, the incoming ball takes a great effect in your shot.
If you stand in and hit firmly, you guide the ball, regardless of where it came from or what kind of spin it has.

also, the speed of the swing will lessen the effect of the angle of reflection.

so if he swings slow while aiming at back of the ball, he's probly going to hit it out.

i do like how the coach told him to control the angle of the racket to control the direction of the ball. a lot of people don't do that well or arn't aware of the angle of their racket when making contact

Thanks for all the replies. I did some digging on the Internet last night and found a great article about this topic. I highly recommend that you guys download it and read every single word in it. The article is by Professor Brody and is titled “Unforced errors and error reduction in tennis” http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2577482/pdf/397.pdf. I copied the relevant portion below.

Changing angles
However, an opponent will quickly catch on if every shot is returned to where it came from. A player who knows the facts about ball/racket interaction can reduce the errors that may occur even when changing the ball angle. If the ball is not going to be hit hard, it should be aimed a little closer to the centre of the court. With a hard swing, the shot can be aimed closer to the sideline or the corner with confidence.
The famous statement that the angle of reflection equals the angle of incidence holds for light reflecting from a plane mirror, but not for tennis balls rebounding from a racket.
Often, in a match, players ease up when well ahead and do not hit shots quite so hard. This can reduce the errors of depth, but can also lead to a problem. If the ball is still aimed the same way, but the swing is no longer as hard, balls that previously went down the line may now end in the alley.
A similar problem can result from changing the game plan in the middle of a match. A player may become concerned about the final outcome, so instead of hitting out and playing his or her regular game, may decide to play it safer and ease up on the strokes. Again, balls that previously went down the line may now end in the alley. The player ends up making more, not fewer errors. People will claim that the player “choked”, but what actually happened is that they did not understand the laws of physics (fig 1).

As the caption says, it describes the angle at which the ball comes off the racket when hitting a relatively hard hit ball at 20 degrees off racket hitting surface axis. 60 ft/sec is about 41 mph which means the opponent hit the ball initially at 91 mph (this is based on an assumption that the ball slows down to 45% of initial speed-check advancedtennis.com if you are curious).
What this tells me is that even if you are hitting a relatively hard DTL shot (at greater than 30 ft/sec=20 mph), the racket face should still point 5-10 degrees inward from the sideline whether you do this consciously or not. Otherwise, the ball will land in the alley.